The present invention relates to a buried storage tank with a single fluid-tight vessel or enclosure for the confinement of any fluid whatsoever such in particular as a liquefied cryogenic gas. More particularly the invention relates to a storage tank for the confinement of a liquid natural gas, for example for a cargo loading harbour terminal.
There has already been proposed mainly in the case of plants or equipments founded or established on a relatively loose ground to at least partially bury a tank for warehousing or "storing" a fluid such for instance as a liquid fuel. Such a solution permits with respect to conventional "elevated" tanks to reduce the surface area at the ground and to limit the impact upon the environment of the equipment while offering a high safety level.
There in particular exists one type of buried tank comprising an external concrete vessel or enclosure buried into the ground and provided to allow the excavation into the latter of a cavity of corresponding shape. A rigid structure is provided within the external vessel or enclosure. This structure, which is made from reinforced concrete or from metal comprises a bottom slab or flooring bed and a covering cupola or dome. A fluid-tight heat-insulating envelope is fastened inside of the rigid structure and defines within the latter a space where the fluid to be "stocked" or stored may be confined. Generally the rigid structure consists of a second internal reinforced concrete enclosure or vessel wherein the bottom slab which is formed of a cast flooring bed is set in.
Such a tank comprising an external enclosure and an internal enclosure is wearisome to be made and proves to be complex and expensive.
Moreover in the case of plants on the coast or shore or which may be submersed, the water contained in the ground may exert very great forces upon the rigid structure of the conventional buried tanks and in particular upon their bottom slabs or flooring beds since the external enclosure does not permit to properly isolate the inside of the tank from water seepage. Thus with a conventional buried tank with a capacity of the order of 100,000 m.sup.3, it is frequent that the flooring bed be subjected under the effect of the water contained in the ground to a lifting force of the order of 40 t/m.sup.2. Then in order to provide for the stability and the holding of the tank, the combined action of the mass of the latter and of the friction essentially between the flooring bed, the concrete enclosures and the ground should correspond nearly to that of a mass of 100,000 t.
Furthermore the conventional buried tanks should be provided while being built with devices for pumping the water with a substantial flow rate underneath their flooring bed to allow the excavation to be kept dry.